In recent years, e.g., in homes, there are being popularised systems in which plural AV equipments are connected through digital interface to carry out transmission of digital data such as music information or video information, etc. or record such data. For example, there have been developed AV equipments such as video camera, and Digital Versatile Disk (DVD) (Trade Name) player, etc. having interface of IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 1394 high performance serial bus standard (hereinafter simply referred to as IEEE 1394 serial bus) which is digital bus.
Meanwhile, ordinarily, cinema data, etc. are information having copyright, and it is therefore necessary to prevent an unjust copy by user, etc.
In order to prevent unjust copy by user, etc. e.g., in Mini Disk (MD) (Trade Name) system, there is used a method called SCMS (Serial Copy Management System). SCMS  data is information caused to undergo transmission along with music data by digital interface. Such information indicates that music data is any one of data of copy free, copy once allowed and copy prohibited. In the case where mini disk recorder receives music data from digital interface, it detects SCMS, whereby when SCMS is copy prohibited, it does not record music data onto mini disk, when SCMS is copy once allowed, SCMS information is changed into copy prohibited and such recorder records SCMS data together with received music data, and when SCMS is copy free, SCMS information is caused to be as it is and such recorder records SCMS information along with received music data.
In a manner stated above, in the mini disk system, SCMS is used to prevent that data having copyright is unfairly copied.
Moreover, in such a data transmission system to carry out transmission or recording of digital data such as music information or video information, etc. through digital interface, there is proposed a system of carrying out transmission in the state where duplication control information is stored in packet header of data packet on the transmission path.
This duplication control information is defined by, e.g., 2 bits as follows.
00: No copy limitation
10: Only once copy is allowed
01: Copy is prohibited more than that
11: Copy is primarily prohibited
Recording equipment which has received data packet tests (checks) duplication  control information in recording data, whereby when the duplication control information indicates “01” or “11”, i.e., copy prohibit, the recording equipment does not carry out recording of received data. Moreover, when the duplication control information indicates “10”, i.e., copy once allowed, the duplication control information is changed into “01”, i.e., changed into prohibit more than that, whereupon the recording equipment records received data with respect to recording media.
In a manner stated above, generation of copy generated from original data is limited.
Further, in order to have compulsory force with respect to the limitation system of the copy generation, there is also used a method of ciphering data to carry out transmission thereof, whereby only maker which makes a contract to manufacture only equipments that observe or obey the copy generation limitation system is allowed to license information necessary for cryptography and decode operation, etc.
Meanwhile, in the system of transmitting data packet on transmission path in the state where duplication control information is stored in the packet header thereof, in the process where packet is transmitted from the transmitting equipment to the receiving equipment, there is the possibility that duplication control information may be altered by any other equipment.
For example, as shown in FIG. 1, even if duplication control information of packet header of data packet is transmitted from the data transmitting unit 1 side at “11” indicating meaning of copy prohibit, when there is duplication control information altering attack during transmission, and the duplication control information is altered into  “10” indicating once copy permission, the data receiving unit 2 side which has received this packet cannot recognize that this data is primarily copy-prohibited. Since duplication control information of packet header allows once recording, data would be recorded.
As described above, in the conventional data transmission method, there is the possibility that copy generation management cannot be made.